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ant.   Then, solving the' triangle formed by these vectors,
we obtain
a&cos«                     (ID
where a, 6, and c are the magnitudes of a, b, and ct respectively, while 0 and 6 are the angles b and c make with a, Equation (II) determines the magnitude and equation (III) the direction of c.
FIG. 5.
Special Cases,   (a) If a and b have the same direction, as in Fig. 6a, then <t> = 0.   Therefore
tan 6 = 0,
$ » 0.
Thus c has the same direction as a and b, while its magnitude equals the arithmetical sum of their magnitudes.
(b) When a and b are oppositely directed, as in Fig. Ob, ^ = TT.   Therefore
and        tan 0 = 0,
0 = 0.